Electrostatographic printing methods, such as xerography, involve creation of an electrostatic latent image on a charge receptor, such as a photoreceptor. As is well known, in such apparatus, the photoreceptor is imagewise discharged in a manner conforming to an image desired to be copied or printed, and then this latent image is developed with toner. The developed toner image is in turn transferred to a print sheet, which is then fused to fix the transferred toner image thereon.
Charging involves contact charging of a photoreceptor by a bias charge roll (BCR). Its main advantage is its low footprint. Thus it is particularly suited for charging small diameter organic photoconductive drums used in low and mid-volume B/W and color machines. Conventional BCR charging is based on a DC-offset AC excitation waveform. As a result a stable V-hi controlled by the DC bias is achieved when Vpp, the AC peak to peak voltage, is greater than a threshold voltage, V-th. Print quality considerations such as background disappearance and halftone uniformity require Vpp and IAC somewhat greater than the threshold values. Moreover, the trend toward increasing process speed in organic photoconductive drum based machines particularly in tandem color applications leads to even higher AC current requirements.
As is well established, the main drawback of conventional AC BCR charging is the significant limitation it imposes on photoreceptor life because degradative AC corona species are generated in close proximity to the photoreceptor surface. Significant work has been done to extend photoreceptor life such as the development of hard photoreceptor overcoats and corona resistant charge transport layer materials (e.g., PTFE filled charge transport layers) as well as a variety of excitation waveforms such as DC, clipped AC or pulsed bias waveforms, each with varying degrees of success. DC BCR charging is a very effective means of improving wear life, but BCR sensitivity to contamination by toner and photoreceptor degradation products generally precludes its practical use. Pulsed bias and clipped AC excitation waveforms have been shown to greatly improve photoreceptor wear life but a stable V-hi cannot be attained with the latter. Instead V-hi increases monotonically as V-pp and IAC increases. Thus practical implementation would require complex controls to achieve V-hi stability especially across environmental conditions, and may be difficult to achieve.
As hereinbefore discussed, the properties of the charge receptor, such as a photoreceptor, are clearly very important to the overall functioning of a printing apparatus, and to the ultimate quality of images created therewith. The electrical stresses placed on a photoreceptor, with the printing of thousands of images therewith contributes to the degradation of the photoreceptor. As the photoreceptor degrades the quality of images that can be created therewith degrades as well. Thus, in practical embodiments of xerographic printers and copiers, it is inevitable that the photoreceptor will have to be periodically replaced. Replacement of the photoreceptor represents a large expense. It is therefore desirable to provide a method and system by which the photoreceptor, even a pre-existing photoreceptor, can be extended significantly.
In the prior art, U.S. Pat. Nos. 5,543,900 and 5,613,173 disclose a novel type of charging apparatus for use in charging the photoreceptor in a xerographic printer. In combination with the bias roll which initially charges the photoreceptor is a special “clipping” circuit comprising a diode and resistor. The clipping circuit has the function of clipping an oscillating voltage applied to the bias roll, and in turn to the photoreceptor, as the bias roll charges the photoreceptor. The long-term effect of this clipping is that lesser electrical stresses are experienced by the photoreceptor with extended use, and in turn the degradation of the photoreceptor is slowed down.